1. Field of the Invention
The invention relates to processes for converting oxygenates to olefins, particularly to ethylene and propylene. More particularly, the invention relates to processes wherein the ratio of ethylene to propylene is increased.
2. Background of the Art
Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2-C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1-C4 alkanols, especially methanol and ethanol; C2-C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol or dimethyl ether (DME), into one or more olefins involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Variations in this process have included increasing reaction temperature, adjusting dilution level, modifying the crystalline molecular sieve catalyst composition, and pretreating the molecular sieve with an oxygenate or an olefin. These modifications are intended to increase yield and/or alter selectivity toward particular end products.
In spite of many technological advances in converting oxygenates to olefins, however, there remains a need to further increase the quantity of light olefins in the conversion product. In particular, there remains a need to increase product selectivity to ethylene and propylene, and particularly to ethylene.